The present invention relates to a process for concentrating the fiber derived dissolved and colloidal substances of recyclable process waters that come from mechanical pulping.
During mechanical pulping, fiber derived dissolved and colloidal substances (DCS) are released from wood into the process water, the quantity and the composition of the substances depending on the wood raw material and method used. A considerably larger amount of DCS is dissolved from refiner mechanical pulp than from groundwood, but the composition of the DCS in each is almost the same. Two to five % of wood material is released into the process water in mechanical pulping and the subsequent stages of treatment. Because there are no significant chemical changes, the same organic substances are found in the process water as in wood. More than 40% of the DCS released from unbleached spruce thermomechanical pulping (TMP) are carbohydrates. In addition, lipophilic extracts (14%), watersoluble lignans (7%), and low molecular weight acids (2%) are released. The rest consists of, among others, lignin, bark derived phenols, proteins, colloidal fiber fines, and inorganic salts. The majority of carbohydrates dissolved from unbleached spruce TMP consists of acetylated galactoglucomannans (60%). In addition, smaller amounts of arabinogalactanes (11%), arabinoglucuronoxylans (2%), and pectins (2%) are dissolved.
In traditional mechanical pulping methods, production lines are generally integrated with paper manufacturing that uses process water, containing dissolved components obtained from the mechanical grinding process. Generally, the dissolved components are not removed; instead they are recycled in the process water circulation. Some of the DCS are removed from the process along with the paper, but other DCS remain in the circulation water. These remaining DCS can react with paper chemicals and cause technical problems. When water circulation is closed, various DCS are enriched into the water in various ways. The substances with lowest solubility and those that are most easily adsorbed by fibers are removed along with the paper. For example, the proportion of lignin and lipophilic extracts in the total amount of DCS decreases when the DCS level of the circulating water increases, but the proportional part of carbohydrates, instead, increases. In the further treatment of fibers, it can be technically difficult to use process water that contains dissolved and colloidal components. Process water can reduce the runnability of the paper machine or increase the organic loading of the process, whereby external purification is needed. Such methods of purification include evaporation, deposition, clarification, flotation or some filtration methods. The pure water obtained from concentration can be returned to circulation. Evaporation is a common method in the pulp industry but not so much in paper processes yet. The treatment of process water containing polymeric carbohydrates by concentration is difficult because of colloidal substances and viscous proper ties.
FI patent publication 85 041 suggests decomposition of ingredients dissolved or dispersed from paper pulp, with the aid of enzymes, by particularly adding enzymes that break down hemicellulose to the water circulation of a paper machine. The purpose was to improve infiltration from the pulp track formed on the wire and to improve the retention of fibers. FI patent publication 954377 describes the use of enzymes, e.g., hemicellulases, in detaching compounds derived from wood or coating which adhere to a moving member of the paper machine. FI patent publication 90 670 relates to hydrolyzing with pectinase the detrimental pectin dissolved from pulp during alkali treatment or peroxide bleaching, whereby the need to add cationic polymers is decreased. U.S. patent publication 5,415,735 suggests treatment of the glucomannans of the DCS by esterases to redeposit the glucomannans on the surface of the fibers and to reduce the concentration of dissolved substances. FI patent publication 93 230 deals with depositing resinous substances onto a fiber by hydrolyzing with hemicellulases the carbohydrates that are on top of colloidal pitch droplets in the process water, whereby the stability of the pitch droplets deteriorates and the resinous substances are deposited onto the surface of the fibers. However, none of the publications mentioned above tries to solve the problem with concentration of the process waters derived from pulping.
Terho and Malinen (1996) (Wet-End Chemistry in Closed-Cycle Papermaking. International Conference on New Available Techniques; 5th International Conference on New Available Techniques, 1. World Pulp and Paper Week (SPCI): 525(1996;SPCI): Paperchem AB16849) and Gullichsen (1993) (Towards the Nonpolluting Pulp Mill; General Aspects, EUCEPA International Environmental Symposium: Pulp and Paper Technologies for a Cleaner World (EUCEPA), Session: Plenary Lectures: 27-34 (1993:CTP) Dialog File: 240 Paperchem AB6714168) deal with concentrating the process water from mechanical 5 pulping by evaporation.
The purpose of the present invention is to eliminate the drawbacks of known technology and to provide a new kind of method to enhance the concentration of DCS in the process waters derived from mechanical pulping.
In the process according to the invention, the concentration of DCS can be enhanced by using enzymes, whereby a greater amount of recyclable water can be separated from the concentrate. In this case, the circulation water coming to the paper machine contains a smaller amount of detrimental substances. The purpose of the invention in particular is to decrease the amount of waste water from pulping processes and paper machine circulation waters.
It is well-known that the amount of waste water from pulping processes could be decreased by concentrating the process water. However, when process water is concentrated, the viscosity of the process water tends to go up so high that concentration cannot be continued. By concentrating, a dry content of only 40% can be obtained, whereby it is not economic to burn the concentrate. In the process according to the invention, the increase in viscosity, when process water is concentrated, is prevented by using an enzymatic method or the viscosity of process water that has already been concentrated is reduced, whereby higher dry contents can be achieved as a consequence of concentration. With the aid of the process, dry contents of more than 40%, preferably more than 45%, more preferably more than 50%, and most preferably more than 60% can be achieved, whereby it is profitable to after-treat the concentrate by burning. Because their volumes decrease and their treatment either on dumping areas or by burning becomes easier, the process provides a more economic treatment for concentrates.
The invention provides considerable advantages. Therefore, it is possible to modify the DCS of process water by using enzymatic methods so that the process water can be concentrated by, for example, evaporating it into a high dry content without an increase in the viscosity of the process water, and the majority of the process water can be returned to circulation as pure water. After concentration, the dry content of the concentrate is preferably more than 60%. Because of the reduction in the volume of the concentrate, it is easier to process further. The concentrate can be burned or taken to a dumping area.